Not that uncommon for Content creators and other people who've played for like 1500-2000 hours, maybe even less time since the addition of custom difficulties.

Yeah, custom difficulty had some wild combos in the past. You could get some good experience fairly reliably.

Even now, if you pick a cursed item that helps you pinpoint the ghost room (say, ouija board or mirror), and crank up the hunt difficulty but set yourself up to (hopefully) figure out the ghost before it ever needs to hunt, you can get ~200-300 experience per run, and each one should be fast (before a hunt, so usually around 5 minutes).

If you plan on farming experience/cash, you can churn out quite a lot in a very short amount of time, especially if you have experience. Might not be the most fun way to play, minimal challenge, and the focus is on volume (high payout over time).

I mean... setting it to max difficulty, largest map, and just guessing the ghost (leave the moment you arrive) can get you 15 levels per success.
That's not fun to play, and more like pulling a slot machine arm over and over with the eventual payouts being "worth it".

If someone wanted to get 10s of thousands of levels, it wouldn't take them that long honestly. A couple months probably. Boring as hell though.
Content creators that have been playing for literal years will have them through massive amounts of playtime. I don't have that kind of staying power, but then I'm not making money of my playtime.

The current highest level legitimate player that I know of is Shuee, at a monumental level 43,000+ with 7,300+ hours played as of about 30 days ago. (Likely closer to 45,000 and 7600 by now)

Shuee has played a lot, and I do mean a LOT since the game came out (an average of 6-8hrs 5 days a week, ish, is what I concluded last time I did the math.)

She never explicitly grinded for levels using custom difficulty either, from what I recall. Just consistent time investment and accrued skill enabling her to win often and in less time per match.